feat: notify users when their phone number has changed

[issuedat]

This PR adds support for sending the user an email notification when their phone number has been changed.

3 new environment variables are introduced:

• GOTRUE_MAILER_SUBJECTS_PHONE_CHANGED_NOTIFICATION: Email subject to use for phone changed notification.
• GOTRUE_MAILER_TEMPLATES_PHONE_CHANGED_NOTIFICATION: The URL to specify a custom template.
• GOTRUE_MAILER_NOTIFICATIONS_PHONE_CHANGED_ENABLED: whether the notification is enabled or not.

The feature is disabled by default. To enable it, the GOTRUE_MAILER_NOTIFICATIONS_PHONE_CHANGED_ENABLED environment variable must be set to true.

[coveralls]

Pull Request Test Coverage Report for Build 18161668709

Details

• 29 of 47 (61.7%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 67.598%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/api/verify.go	10	13	76.92%
internal/api/mail.go	9	15	60.0%
internal/mailer/templatemailer/templatemailer.go	0	9	0.0%

Totals
Change from base Build 18138525531:	-0.02%
Covered Lines:	13381
Relevant Lines:	19795

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💛 - Coveralls

[cstockton]

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